In oxidative phosphorylation, what is the primary purpose of the electron transport chain (ETC)?

• A. To import pyruvate into the mitochondrial matrix for conversion to acetyl-CoA
• B. To transfer electrons from NADH and FADH${}_2$ to O${}_2$ while pumping protons to generate a proton-motive force used to make ATP
• C. To directly catalyze phosphorylation of ADP to ATP by substrate-level phosphorylation
• D. To synthesize NADH from NAD$+$ using energy from ATP hydrolysis

Answer: (B)

Verified step by step guidance

1. Understand the role of the electron transport chain (ETC) in oxidative phosphorylation, which is a process occurring in the inner mitochondrial membrane.
2. Recognize that the ETC transfers electrons from electron carriers NADH and FADH\_2 to molecular oxygen (O\_2), the final electron acceptor.
3. Note that as electrons move through the ETC complexes, energy is released and used to pump protons (H\^+) from the mitochondrial matrix into the intermembrane space, creating a proton gradient.
4. This proton gradient, also called the proton-motive force, stores potential energy across the inner mitochondrial membrane.
5. Finally, the proton-motive force drives ATP synthesis by ATP synthase, which uses the flow of protons back into the matrix to convert ADP and inorganic phosphate into ATP.